Inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath width" of the pen, the maximum pattern of ink which can be laid down in a single pass. The media is moved through the printzone, typically one swath width at a time, although some print schemes move the media incrementally by for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
An automatic manner of sensing the size of print media which has been loaded at the input of an inkjet printing mechanism is the subject addressed herein. The print media, may be any type of substantially flat material, such as plain paper, specialty paper, card-stock, fabric, transparencies, foils, mylar, etc., but the most common type of medium is paper. For convenience, we will discuss printing on paper as a representative example of these various types of print media. The media may be supplied to the printing mechanism in a variety of different sizes. For instance, in desktop inkjet printers, paper is typically supplied in a stack of cut-sheets, such as letter size, legal size, or A-4 size paper, which are placed in an input tray. Smaller sized envelopes or postcards or other media sizes may also be used for printing. Typically, the media sheets, cards or envelopes are sequentially pulled from the top of the stack and printed on, after which they are deposited in an output tray.
It would be desirable to have an inkjet printing mechanism which can communicate to the host computer what size of media has been loaded into the printing mechanism, particularly when the printing mechanism is not within sight of the computer operator, such as when several computer users on a networked system share a single printer. Even when an inkjet printer is located on the operator's desktop, it may be helpful to provide the operator with a warning if the wrong size media is loaded for a particular print job. This would allow the operator to load the proper size media, or to adjust the print job parameters to fit the size of the loaded media.
Sensing of media length and width is a feature often found in high-end printers and plotters for business and industrial use, where there is less sensitivity to price. Unfortunately, for the small business and home markets, automated media type detection has not been economically viable. The expense of the earlier methods of media sensing precludes their use within the frugal, highly competitive home market segment.
In the past, two basic methods have been used for media size determination. The first method employed a retroflective photo diode attached to the carriage of the printer or plotter. In a reciprocating printing mechanism, the carriage carries the inkjet pens while traversing the width of the media to lay down a swath of ink. As the carriage passed over the edge of the media, the signal received from the retroflective sensor correspondingly shifted due to the change in reflectance seen after passing over the media edge. The printer controller then noted the position of the carriage at which the retroflective sensor signal shifted. As the carriage moved in the opposite direction to the other side of the media, the shift was again detected, with the difference between the carriage positions at these two detection points being used by the printer controller to determine the media width.
The second method of determining the media size functions in a manner similar to the first, except that a capacitive sensor is mounted on the carriage for use as a media edge detector. Resolution of either method varies depending upon the quality and focal distance of the sensor. In general, the retroflective sensor delivers a higher resolution for a lower cost, and is consequently more prevalent. However, the capacitive sensor has the advantage of being more robust when detecting transparent media, such as that used to make overhead projector slides.
Besides width, media length detection has also been performed by plotters using retroflective and capacitive sensors. In these systems, with the printhead carriage positioned over the media, the feed roller is driven forward to move the media underneath the carriage-mounted sensor. When the leading edge of the media is detected, the position of the media drive roller is noted. The media is then driven on until the trailing edge of the media is detected, again noting the position of the drive roller. The difference between these two positions is used by the printer controller to calculate the media length.
Unfortunately, this technique of media length detection is used only with plotters, because most inkjet printers are limited to driving media unidirectionally, that is, only forward through the print zone. One exception to this rule of forward driving for printers is the picking routine used to separate the media sheets of a Z-fold media stack, often used to print banners. In this banner pick routine, the backwards drive motion is used merely to separate the media sheets prior to picking, not to determine the location of the trailing edge of the media, and not to determine the media size. Desktop inkjet printers using this banner pick routine include the DeskJet.RTM. 682C and 693C model printers, produced by the Hewlett-Packard Company of Palo Alto, Calif., the present assignee.
Both the reflective photodiode and capacitive sensor methods rely on the position of the carriage to determine the media width. Commonly, carriage position sensing is accomplished using a quadrature optical encoder, although other devices, such as magnetic encoders, linear variable differential transformers, and cable-driven rotary potentiometers have also been used. All of these methods typically achieve a resolution of better than 0.2 millimeters (mm). In a similar manner, media length determination relies on the accuracy of the feed roller position sensor. This sensor is typically a rotary quadrature optical encoder device mounted on the roller drive motor. Rotary quadrature optical encoders normally achieve a resolution similar to that of the carriage position sensors.
Unfortunately, both the retroflective and capacitive sensing techniques require mounting the sensor on the printhead carriage. The extra mass of the sensors, along with that of the associated mounting hardware, must be continually carried by the carriage during operation. This additional weight requires a heavier-duty bearing design, draws greater power from the carriage servo system during use, and may decrease throughput (pages per minute) because the more massive carriage requires more time to reverse directions. Furthermore, additional flexible signal cabling and connections must also be made to the carriage, beyond those required to communicate with the inkjet cartridges for printing. Moreover, mounting these sensors on the printhead carriage requires the carriage to be wider, which then results in the overall product being wider, which increases the footprint of the printer, that is, the desktop workspace space required to house the printer. A larger printer footprint is undesirable to many consumers who require a compact inkjet printing mechanism. Thus, these carriage mounted sensors are inherently expensive, both in terms of direct cost and indirect costs or detriments to most consumers.
Additionally, these earlier retroflective and capacitive sensors are themselves relatively expensive. Various trade-offs typically need to be made between the resolution of the sensors and their expense, although these trade-offs are limited because these sensors must detect the media edges bi-directionally, first when traveling to the left then when traveling to the right. While less expensive sensors could be used, they typically suffer greater hysteresis effects, so accuracy is limited. Furthermore, the less expensive sensors have increasingly small focal distances, so the sensor face must be held very close to the media. Such a small media-to-sensor spacing limits the design margin available for producing economical printers for the home and small business environments.
Finally, since most desktop inkjet printers drive the media only unilaterally forward through the print zone, the length detection technique used in plotters cannot be employed. Furthermore, many printers sold into the home and small business markets have the media driven on an open-loop control system, without any feedback regarding the position of the media drive roller. Thus, typical home printers are incapable of media length detection, although a need for media length size detection, as well as media width detection, clearly exists in both home and small business inkjet printer markets.